Draw a background in photoshop?

Question

Adobe Photoshop

In general, there is a certain size of the frame, it is necessary to fit all the pictures under it, some fit well into the frame, I do it in Photoshop through free transformation. And some have free space around the edges. (picture) Is there any tool or method, maybe in Photoshop or somewhere else, that will help to draw these areas in meaning? Or something else?